3) The idealized structural formula for amphiboles may be defined as: ABC(OH. Th

Question

3) The idealized structural formula for amphiboles may be defined as: ABC(OH. The B- and C-sites of amphibole, which have octahedral geometries (M-site; coordination number of 6), may accommodate elements as diverse as Fe, Mg, Ca and Na* in the B-site, and Tit, Als, Fe, Fe and Mg2 in the C-site. By contrast, the octahedral (M-sites) in the most common rock-forming pyroxene minerals (MIM2Si:O%) may only accommodate Fe? Mg and Ca. Describe why, using the sketches prepared for question 2 (if necessary), which properties of the amphibole structure enable accommodation of such a broad diversity of ionic species, and the pyroxene structure cannot (6 Marks). 4) An amphibole in an outcrop of intermediate igneous rocks has the following composition: If, during metamorphism of the igneous rock, the composition of the amphibole becomes more silicon (Si) rich (e-g. the abundance of Si in the T-site increases), explain how the occupancy of the other cation sites may change in order to maintain charge balance? (8 Marks

Explanation / Answer

Answer 1: amphibole structure is similar to the Pyroxene, both are chain silicates, however, the main difference between them is amphibole is double chain silicate and pyroxene is a single chain. This is the main reason why amphibole structure accommodates the broad diversity of anions and cations.

Now you may ask why this structure let the amphibole to behave differently from the pyroxenes. In case of amphibole, the double chains of silica tetrahedra align parallel to the C- axis and stacked in an alternative manner. This geometry will lead to the generation of five different sites between the chains. The name of the sites are Octahedral M1, M2 and M3 sites that lie between points of chains and M4 and A sites that lie between the bases.M4 and A site act as a welding material since it weld one double chain structure of amphibole with another double chain and we also get cleavage plane where M4 and A site weld the two different double chain. Fe2+, Mg, Fe3+, Al occupy all octahedral sites(M1, M2, M3).In case of Pyroxenes, there is only single chain so no stacking of chain unlike amphiboles to form sites like M1 M2 and M3 type rather, in pyroxenes, one single silica tetrahedra chain weld with another single chain via M1 and M2 types ions sites. So Pyroxenes have only two sites(M1 and M2) while amphiboles have five sites(M1 M2 M3 M4 and A) so amphibole is more prone to accommodate different species of ions because of lots of vacancies as compared to Pyroxenes.

Answer 2

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